Tetrafluoroethylene copolymer

ABSTRACT

A polymerization process for producing a copolymer of tetrafluoroethylene and perfluorobutylethylene, prepared by an aqueous dispersion polymerization technique, and the resin produced thereby, are provided. This copolymer contains a relatively small amount, between about 0.02 weight percent and about 0.6 weight percent, of comonomer polymerization units. The copolymer is believed to be comprised of a core-shell structure wherein the polymerized comonomer units reside primarily within the core. The primary particle size of the copolymer ranges from 0.175 microns to and including 0.203 microns and the standard specific gravity is less than 2.143. This copolymer possesses a unique combination of very small particle size coupled with high molecular weight, a combination not heretofore achieved in tetra-fluoroethylene polymers of the dispersion or fine powder type. The process of this invention is characterized by permanganate initiation and the entire reaction is carried out in the absence of zinc chloride or other ionic strength enhancer. The addition of initiator must be stopped well before completion of the reaction, preferably at or before the mid-point of the complete reaction.

BACKGROUND OF THE INVENTION

[0001] The invention relates copolymers of tetrafluoroethylene andperfluorobutylethylene produced by aqueous dispersion polymerization.

[0002] Many prior patents disclose techniques for the dispersionpolymerization of tetrafluoroethylene, and variations thereof. Thedispersion polymerization of tetrafluoroethylene produces what has cometo be known as “fine powder” resins. In such a process, sufficientdispersing agent is introduced into a water carrier such that, uponaddition of tetrafluoroethylene in the presence of a suitablepolymerization initiator and upon agitation, and under autogenoustetrafluoroethylene pressure of 10 to 40 kg./cm², the polymerizationproceeds until the level of colloidally dispersed polymer particles isreached and the reaction is then stopped. See, e.g., U.S. Pat. No.4,016,345 (Holmes, 1977).

[0003] Tetrafluoroethylene powders have also been produced by a processof suspension polymerization, wherein tetrafluoroethylene monomers arepolymerized in a highly agitated aqueous suspension in which little orno dispersing agent may be employed. The type of powder produced insuspension polymerization is termed. “granular” resin, or “granularpowder”. See, e.g., U.S. Pat. No. 3,655,611 (Mueller, 1972).

[0004] For both types of polymerization processes, copolymerization oftetrafluoroethylene with various fluorinated alkyl ethylene comonomershas been described. See, for example, U.S. Pat. No. 4,792,594 (Gangal,et al., 1988). The present invention relates to the aqueous dispersionpolymerization technique wherein the product of the polymerizationreaction is the copolymer of the invention dispersed within an aqueouscolloidal dispersion. This process, generally, is one in whichtetrafluoroethylene monomer is pressured into an autoclave containingwater and certain polymerization initiators along with paraffin wax tosuppress coagulum formation and an emulsifying agent. The reactionmixture is agitated and the polymerization is carried out at suitable,temperatures and pressures. Polymerization results in the formation ofan aqueous dispersion of polymer. The dispersed polymer particles may becoagulated by techniques known in the art to obtain fine powder polymer.When fluorinated alkyl ethylene comonomers are introduced into thepolymerization, it is known that the comonomer reacts much faster thantetrafluoroethylene monomer, and comonomer addition rate is important tothe distribution of comonomer achieved in the copolymer. When thiscomonomer is added as a single precharge, the comonomer is found inpolymerized form mostly in the core or interior of the polymerparticles. The comonomer may also be injected through some or all of thepolymerization process and the injection sequence determines thestructure of the shell, i.e., if comonomer is added throughout, it willreside throughout the outer shell of each copolymer particle.

[0005] Various prior patents have disclosed variations on techniques forthe homopolymerization of tetrafluoroethylene and for thecopolymerization of other monomers with tetrafluoroethylene. Among thoseare included U.S. 4,576,869 (Malhotra, 1986) and U.S. Pat. No.6,177,533B1 (Jones, 2001). Within those references are contained certainprocedures which have become, more or less, accepted procedures fordetermining certain defining and delineating properties associated withtetra-fluoroethylene homopolymers and copolymers. Among those propertiesare:

[0006] (a) the Standard Specific Gravity (SSG), measured by waterdisplacement of a standard molded test specimen, in accord with ASTMD-1457-90;

[0007] (b) Raw Dispersion Particle Size (RDPS), determined byspectrophotometry or other suitable technique. See, e.g., U.S. Pat. Nos.4,016,345 and 4,363,900. The measurements herein were obtained by laserlight scattering using a Brookhaven 90 plus instrument.

[0008] In the cited prior patents, and almost universally, the SSG of ahomopolymer specimen has come to define its molecular weight, with therelationship being inverse, that is, a high molecular weight (MW)corresponds to a low SSG and, generally, the lower the SSG, the higheris the molecular weight. Addition of comonomer into the polymerizationprocess may also reduce SSG and, for resins modified with comonomer, SSGmay be used to infer variations in molecular weight at a given constantcomonomer level.

[0009] For fluoroethylene fine powder polymers, generally, their RDPSrange from about 0.175 microns and below to about 0.325 microns. Thesefine powder resins are known to be useful in paste extrusion processes,and in stretching (expansion) processes in which the paste-extrudedextrudate, after removal of extrusion aid lubricant, is stretchedrapidly to produce porous, strong products of various cross-sectionalshapes such as rods, filaments, sheets, tubes, etc. Such a stretchingprocess is disclosed in U.S. Pat. No. 3,953,566 (Gore, 1976), assignedcommonly with the instant invention.

[0010] Heretofore it has generally been accepted that, fortetrafluoroethylene homopolymers and copolymers of the dispersion type,it is difficult to achieve a resin which combines both desirableproperties of small particle size (RDPS) coupled with a high molecularweight (MW). Expressing the same conclusion in a different, equivalentway, it is generally accepted that a dispersion resin possessing a smallraw dispersion particle size (RDPS) and a low standard specific gravity(SSG) has been difficult or impossible to achieve.

[0011] This invention provides a dispersion type copolymer oftetrafluoroethylene and perfluorobutylethylene comonomer which possessesa heretofore unachieved combination of small fundamental resin particlesize (RDPS) coupled with a low SSG (high MW).

SUMMARY OF THE INVENTION

[0012] A polymerization process for producing a tetrafluoroethylenecopolymer, and the copolymer produced thereby, are provided. Thecopolymer is of the dispersion/fine powder type and contains polymerizedtetrafluoroethylene monomer units and co-polymerizedperfluorobutylethylene comonomer units in which the primary particlesare believed to have a core and shell structure and the polymerizedcomonomer units are present in an amount from 0.02% by weight to 0.6% byweight, based upon total copolymer weight. The copolymer has a rawdispersion primary particle size (RDPS) in the range between 0.175microns to and including 0.203 microns coupled with a standard specificgravity (SSG) of less than 2.143. Preferably the copolymer has comonomerunits present in an amount from 0.05% by weight to 0.5% by weight andthe RDPS is within the range between 0.178 microns and 0.200 micronscoupled with a SSG of less than 2.140.

[0013] The copolymer may be dispersed within an aqueous dispersionand/or may be in the form of fine powder.

[0014] The process of the invention is characterized in that thecopolymerization reaction is catalyzed by potassium permanganateinitiator and the entire reaction is carried out in the absence of anymultivalent ionic strength enhancer, such as zinc chloride. The additionof initiator is stopped well before completion of the reaction,preferably at or before the mid-point of the complete reaction. Also,and preferably, the comonomer is added as a precharge into thecopolymerization reactor, although it may be added incrementally andintermittently through a portion of the polymerization reaction process.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

[0015] A copolymer of tetrafluoroethylene and perfluorobutylethylene,prepared by an aqueous dispersion polymerization technique, is provided.This copolymer contains a relatively small amount, between about 0.02weight percent and about 0.6 weight percent, of fluorinated comonomerpolymerization units. The copolymer is believed to be comprised of acore-shell structure wherein the polymerized comonomer units resideprimarily within the core. The primary particle size of the copolymerranges from 0.175 microns to and including 0.203 microns and thestandard specific gravity is less than 2.143. This resin is uniquelysuited for use in an expansion (stretching) process, to produce highstrength, highly porous tetrafluoroethylene polymeric articles.

[0016] The polymers of this invention provide the heretofore unachievedcombination of properties wherein the fundamental particle size is verysmall and this is coupled with high molecular weight. These polymers areproduced by a dispersion polymerization process, which is described-indetail below and in the examples which follow. It can be seen from thoseexamples, and drawing upon basic principles of dispersion polymerizationof tetrafluoroethylene monomers, in particular, certain processing stepsdisclosed herein are critical.

[0017] More specifically:

[0018] Initiation of Polymerization

[0019] The copolymer of this invention is produced by a polymerizationprocess wherein the copolymerization reaction is catalyzed by apermanganate initiator, preferably potassium permanganate (KMnO₄), inthe absence of any multivalent ionic strength enhancer, and theinitiator addition is stopped completely, allowing the reaction to slowdown and proceed to completion, at a point between 30% and 80% of theprogression of the reaction toward completion. Preferably the initiatoraddition is stopped at about the mid-point of the reaction, i.e., at40-65% to completion, and most preferably at about 44% of the completionof the reaction.

[0020] The perfluorobutylethylene comonomer is preferably added as aprecharge in the reaction or, alternatively, it can be addedincrementally only through a portion of the reaction.

[0021] Dispersing Agents

[0022] Substantially non-telogenic dispersing agents are used. Ammoniumperfluoro octanoic acid (APFO or “C-8”) is an acceptable dispersingagent. Programmed addition (precharge and pumping) is known and ispreferred. Decreasing the precharge can lead to increased primaryparticle size.

[0023] Polymerization Control

[0024] It is known that ionic strength affects primary particle sizecontrol and dispersion stability. Care must be taken to have asufficiently stable dispersion to enable completion of thepolymerization without coagulating the dispersion and to have asufficiently stable dispersion to survive transportation from the,polymerization vessel-to the coagulator. Inorganic salts have beenprecharged into the polymerization reactor with the intended effect ofincreasing the primary particle size. Multivalent ions, generally, aremore effective in increasing ionic strength. Zinc chloride has beenemployed together with decreased APFO and intended to control (increase)the primary particle size. In the polymerization reaction of the presentinvention, however, multivalent ionic strength enhancers, such as zincchloride, are omitted from the reaction.

[0025] It is known that particular attention must be paid to ingredientpurity to achieve the desired properties in polymerizations as describedherein. Ionic impurities, which can also increase ionic strength, inaddition to soluble organic impurities, which can cause chain transferor termination, must be minimized. It is clearly important to employultra pure water in all such polymerization reactions.

[0026] Additional Test Procedures

[0027] The break strength associated with an extruded and expanded(stretched) beading produced from a particular resin is directly relatedto that resin's general suitability for expansion, and various methodshave been employed to measure break strength. The following procedurewas used to produce and test expanded beading made from the copolymersof this invention:

[0028] For a given resin, blend 113.4 g. of fine powder resin togetherwith 32.5 ml. of Isopar® K. Age the blend for about 2 hours at 22° C. ina constant temperature water bath. Make a 1 in. diameter cylindricalpreform by applying about 270 psig of preforming pressure for about 20seconds. Inspect the preform to insure it is crack free. Produce anextruded beading by extruding the preformed lubricated resin through a0.100 in. diameter die having a 30 degree included inlet angle. Theextruder barrel is 1 in. in diameter and the ram rate of movement is 20in./min. The extruder barrel and die are at room temperature, maintainedat 23° C., plus or minus 1.5° C. Remove the Isopar K from the beading bydrying it for about 25 minutes at 230° C. Discard approximately thefirst and last 8 ft. of the extruded beading to eliminate end effects.Expand a 2.0 in. section of the extruded beading by stretching at 290°C. to a final length of 50 in. (expansion ratio of 25:1) and at aninitial rate of stretch of 100% per second, which is a constant rate of2 in. per second. Remove about a 1 ft. length from near the center ofthe expanded beading. Measure the maximum break load of the sample atroom temperature (23° C. plus or minus 1.5° C.) using an an Instron®tensile tester using an initial sample length of 2 in. and a crossheadspeed of 12 in./min. Measure in duplicate and report the average valuefor the two samples. This procedure is similar to that described in U.S.Pat. No. 6,177,533B1. The expansion here is carried out at 290° C.instead of 300° C.

EXAMPLES Example 1

[0029] To a 50-liter, horizontal polymerization reactor equipped with a3-bladed agitator was added 1.5 kg. paraffin wax, 28 kg. of de-ionized(DI) water, 18 g. of ammonium perfluoro-octanoic acid (APFO) and 5 g. ofsuccinic acid dissolved in about 50 grams of DI water. The reactor andcontents were heated above the melting point of the wax. The reactor wasrepeatedly evacuated and pressurized (to about 1 atmosphere or less)with TFE until the oxygen level was reduced to 20 ppm or less. Thecontents were briefly agitated at about 60 rpm between evacuation andpurge cycles to ensure that the water was deoxygenated. To the evacuatedreactor under vacuum were added 8 ml. of PFBE as a precharge ofcomonomer, and the reactor was heated to 83° C. TFE was then added tothe reactor until the pressure reached 2.8 Mpa, 3.0 kg., and KMnO₄ in DIwater solution (0.063 g/L) was injected at 80 ml./min. until about 2.0kg. of TFE had been added. This was accomplished in about 7 minutes.About 320 ml. of 20% APFO solution was added in 40 ml. increments, thefirst increment being added after about 1 kg. of TFE had been added tothe reactor, and the subsequent increments added after each subsequentkg. of TFE had been added, so that the final increment was added afterabout 9 kg. of TFE had been charged. The KMnO₄ addition rate wasdecreased to 40 ml./min. at the 2 kg. TFE level and continued at thisrate until about 3 kg. TFE had been added. The KMnO₄ addition rate wasthen further decreased to 20 ml./min. until about 5 kg. of TFE had beenadded. The KMnO₄ addition was then decreased to 10 ml./min. and additionwas continued at this rate until about 7 kg. of TFE had been added tothe reactor, at which time the addition of KMnO₄ was stopped.

[0030] The polymerization reaction was then allowed to continue and thereaction was stopped after about 16 kg. of TFE had been added to thereactor. The weight of the dispersion produced was 46.7 kg. and thedensity of the dispersion was 1.246 gm./ml. (35.0 wt. % solids).

[0031] No KMnO₄ was thus added after 44% of the TFE had been reacted.The dispersion was coagulated and dried at 170° C.

[0032] The raw dispersion particle size (RDPS) of the polymer particleswas 0.203 microns and the standard specific gravity was 2.138. As can beseen, the level of comonomer in the polymerized product was 0.07 weightpercent. The break strength of the beading was 7.9 lbs.

Example 2

[0033] The procedures for Example 1 were repeated except that 60 ml. ofPFBE were added as a precharge to the reaction. The KMnO₄ was added inincrements such that KMnO₄ solution was injected at a rate of 30ml./min. until about 2480 ml. had been added, then the rate was adjustedto 40 ml./min. until an additional 1740 ml. had been added, and the ratewas then reduced to 20 ml./min. until an additional 1640 ml. had beenadded, after which the KMnO₄ addition was stopped, at which point 6 kg.of TFE had reacted.

[0034] The polymerization was allowed to continue and the reaction wasstopped after about 16 kg. of PTFE had been added to the reactor. Theweight of the dispersion produced was 49.7 kg. and the dispersioncontained 35.7 wt. % solids.

[0035] The level of comonomer in the reaction product was 0.5 weightpercent.

[0036] The RDPS of the polymer particles was 0.190 microns and the SSGwas 2.135. The break strength of the beading was 9.0 lbs.

Example 3

[0037] The procedures for Example 1 were repeated except that 43 ml. ofPFBE were added as a precharge to the reactor after which additionalamounts of PFBE were added in increments during the polymerizationreaction, a total of 14 ml. additional PFBE being added in 2-7 ml.increments. The first 7 ml. increment of PFBE was added after about 2kg. of TFE had been injected into the reactor, and the second 7 ml.increment was added after 4 kg. of TFE were added.

[0038] The KMnO₄ was added beginning at a rate of about 54 ml./min.,until 2000 ml. had been added, then the rate was reduced to about 50ml./min. until an additional 900 ml. had been added, and the rate wasagain reduced to about 33 ml./min. until an additional 1780 ml. had beenadded, after which KMnO₄ addition was stopped. At this point, 6 kg. ofTFE had reacted.

[0039] The polymerization was allowed to continue and the reaction wasstopped after about 16 kg. of TFE had been added to the reactor. Theweight of the dispersion produced was 50.7 kg. and the dispersioncontained 33.9 wt. % solids.

[0040] The level of comonomer in the reaction product was 0.5 weightpercent. It is seen that the initiation of polymerization was stopped at38% of the complete reaction of the TFE.

[0041] The RDPS of the polymer particles was 0.176 microns and the SSGwas 2.142. The break strength of the beading tested was 11.0 lbs.

Comparative Example A (Omitting PFBE Precharge)

[0042] A polymerization reaction was conducted essentially as describedin Example 3 except that the precharge of 43 ml. of PFBE was omittedand, instead, 49 ml. of PFBE were added in 7-7 ml. increments. The first7 ml. increment was added after 1 kg. of TFE had been added. Subsequentincrements were added after each additional kg. of TFE addition, withthe last increment being added after the addition of the 7^(th) kg. ofTFE.

[0043] The KMnO₄ solution was added initially at a rate of about 54ml./min. until 910 ml. had been added, and the rate was reduced to about20 ml./min. until an additional 400 ml. had been added, and then therate was increased to about 40 ml./min. until a further 1600 ml. hadbeen added, after which the KMnO₄ addition was stopped. At that point, 7kg. of TFE had reacted, and a total of 2910 ml. of KMnO₄ solution hadbeen added.

[0044] The polymerization was allowed to continue and the reaction wasstopped after about 16 kg. TFE had been added to the reactor. The weightof the dispersion produced was 46.3 kg. and the density of thedispersion was 1.244 gm./ml. (34.8 wt. % solids). The dispersion wascoagulated and dried at 180° C.

[0045] The RDPS of the polymer particles was 0.258 microns and the SSGwas 2.145. The break strength of the beading was 7.7 lbs.

Comparative Example B (Excessive Precharge of PFBE)

[0046] The reaction of Example 1 was repeated essentially as describedexcept that 90.0 ml. of PFBE was precharged to the reactor instead ofthe 8 ml. described in Example 1. The KMnO₄ solution was added at a rateof about 53 ml./min., until about 5890 ml. had been added, at whichpoint the experiment was terminated as a result of no reactionoccurring.

Comparative Example C (Omitting PFBE)

[0047] The reaction of Example 1 was repeated essentially as describedexcept that no PFBE was added. The KMnO₄ solution was added beginning ata rate of about 60 ml./min. until 2.0 kgs. of TFE had been added, thenthe rate was reduced to about 40 ml./min. until 5 kgs. of TFE had beenadded, and reduced again to 20 ml./min., after which KMnO₄ addition wasstopped, at which point 9 kg. of TFE had reacted and a total of 1250 ml.of KMnO₄ solution had been added.

[0048] The polymerization was allowed to continue and the reactor wasstopped after a total of 16 kg. TFE had been added to the reactor. Theweight of the dispersion produced was 45.7 kg. and the dispersioncontained 36.7 wt. % solids. The dispersion was coagulated and dried at170° C.

[0049] The RDPS of the polymer particles was 0.284 microns and the SSGwas 2.158. The break strength of the expanded beading test sample was6.9 lbs.

[0050] While the invention has been disclosed herein in connection withcertain embodiments and detailed descriptions, it will be clear to oneskilled in the art that modifications or variations of such details canbe made without deviating from the gist of this invention, and suchmodifications or variations are considered to be within the scope of theclaims hereinbelow.

What is claimed is:
 1. A process for the copolymerization of atetrafluoroethylene copolymer of the dispersion/fine powder type, saidcopolymer containing essentially from 99.4% by weight to 99.98% byweight tetrafluoroethylene monomer units and from 0.02% by weight to0.6% by weight copolymerized perfluorobutylethylene comonomer units,comprising: (a) copolymerizing said monomer and comonomer in apressurized reactor, and (b) initiating said copolymerization by addingpotassium permanganate (KMnO₄), (c) carrying out the entire reaction inthe absence of any ionic strength enhancer, and (d) stopping theaddition of (KMnO₄)initiator at a point in the reaction no further than80% of the reaction completion.
 2. The process of claim 1 includingadding the comonomer as a precharge in the copolymerization reaction. 3.The process of claim 1 including adding the comonomer incrementally andintermittently, from the beginning of the reaction through only aportion of the complete reaction.
 4. The process of claim 1 includingstopping the addition of KMnO₄ initiator at a point in the reaction nofurther than 60% of the reaction completion.
 5. The process of claim 1including stopping the addition of KMnO₄ initiator at a point in thereaction no further than 50% of the reaction completion.
 6. The processof claim 1 for the copolymerization of a tetrafluoroethylene copolymerof the dispersion/fine powder type, wherein: said polymerized comonomerunits are present in an amount from 0.02% by weight to 0.6% by weight,based upon total copolymer weight; and said copolymer has a rawdispersion primary particle size (RDPS) in the range between 0.175microns to and including 0.203 microns and has a standard specificgravity (SSG) of less than 2.143.
 7. The process of claim 6 wherein saidcopolymer has a RDPS in the range between 0.175 microns to and including0.203 microns and has a SSG of less than 2.140.
 8. The process of claim6 including adding the comonomer as a precharge in the copolymerizationreaction.
 9. The process of claim 6 including adding the comonomerincrementally and intermittently, from the beginning of the reactionthrough only a portion of the complete reaction.
 10. The process ofclaim 6 including stopping the addition of KMnO₄ initiator at a point inthe reaction no further than 60% of the reaction completion.
 11. Theprocess of claim 6 including stopping the addition of KMnO₄ initiator ata point in the reaction no further than 50% of the reaction completion.12. A tetrafluoroethylene copolymer of the dispersion/fine powder type,containing polymerized tetrafluoroethylene monomer units, andcopolymerized perfluorobutylethylene comonomer units, wherein: (a) saidcopolymerized comonomer units are present in an amount from 0.02% byweight to 0.6% by weight, based upon total copolymer weight; and (b)said copolymer has a raw dispersion primary particle size (RDPS) in therange between 0.175 microns to and including 0.203 microns and has astandard specific gravity (SSG) of less than 2.143.
 13. The copolymer ofclaim 12 wherein said comonomer units are present in an amount from0.05% by weight to 0.5% by weight.
 14. The copolymer of claim 12 whereinthe RDPS is within the range between 0.178 microns and 0.200 microns andhas a SSG of less than 2.140.
 15. The copolymer of claim 12 dispersedwithin an aqueous dispersion.
 16. The copolymer of claim 12 in the formof fine powder.
 17. A tetrafluoroethylene copolymer of thedispersion/fine powder type, containing polymerized tetrafluoroethylenemonomer units and copolymerized perfluorobutylethylene comonomer units,wherein: (a) said copolymerized comonomer units are present in an amountfrom 0.05% by weight to 0.5% by weight, based upon total copolymerweight; and (b) said copolymer has a RDPS between 0.178 microns and0.200 microns and has a SSG of less than 2.140.
 18. The copolymer ofclaim 17 dispersed within an aqueous dispersion.
 19. The copolymer ofclaim 17 in the form of fine powder.
 20. The copolymer of claim 12shaped into the form of expanded beading having a break strength of atleast 7.9 lbs.
 21. The copolymer of claim 17 shaped into the form ofexpanded beading having a break strength of at least 9.0 lbs.
 22. Thecopolymer of claim 20 having a break strength of at least 11.0 lbs.